Ferroelectric memories have received much recent interest as dense randomly accessible memories although the concept for such memories originated several decades ago. See for example, the following recent review articles: "Ferroelectric Memories" by Scott et al. in Science, volume 246, 1989, pages 1400-1405, "Ferroelectrics for nonvolatile RAMs" by Bondurant et al. in IEEE Spectrum, July, 1989, pages 30-33, and "Ferroelectric Materials For 64 Mb and 256 Mb DRAMs," by Parker et al., in IEEE Circuits and Devices Magazine, January 1990, pages 17-26. Because of their high charge storage capability, they may be integrated more densely than possible even semiconductor memories. They also offer non-volatile memory, that is, data is retained in a ferroelectric memory even if power is interrupted.
In a ferroelectric material, an electric dipole is created by the relative displacement of positive and negative ions, e.g., Ti.sup.4+ and O.sup.2- in PbTiO.sub.3. In ferroelectrics, similarly to dielectrics, an applied electric field E can induce the dipole, which can be expressed in terms of polarization P, as illustrated in FIG. 1, and which in turn is directly related to a surface charge density expressed in units of .mu.C/cm.sup.2. FIG. 1 does not show the initial priming in which the characteristics start from the origin. A saturation polarization P.sub.S corresponds to maximum displacements of the ions. Unlike dielectrics, in ferroelectric materials, the dipole remains after the electric field is removed (remanent polarization P.sub.R). Furthermore, positive and negative electric fields will produce positive and negative remanent polarizations, respectively. A coercive field E.sub.C is required to switch between the two polarizations. A ferroelectric memory array can be fabricated as an array of capacitors having ferroelectric material in the gap of the capacitors. A ferroelectric capacitor, once switched into its up-state or down-state, that is, 0-state or 1-state, stays in that state until switched again. Further, the state of the ferroelectric capacitor can be interrogated by measuring the polarity of the voltage induced on its electrodes by the switched ferroelectric material.
It was early recognized that bulk ferroelectrics were unsatisfactory for memories because the coercivity or switching fields were of the order of thousands of volts per centimeter, causing excessively high switching voltages. However, ferroelectric thin films avoid the problem of large voltages by reducing the thickness of the ferroelectric to obtain the required switching fields E.sub.C with voltages common in semiconductor circuits. As has been disclosed by Rohrer in U.S. Pat. No. 3,728,694, ferroelectric thin film capacitors are made by depositing a metallic lower electrode, depositing the ferroelectric layer on the lower electrode, and then depositing an upper metallic electrode. More advanced techniques for integrating ferroelectric memories with a semiconductor integrated circuit are disclosed by McMillan et al. in U.S. Pat. No. 4,713,157 and by Rohrer et al. in U.S. Pat. No. 4,707,897. For example, in order to integrate semiconductor gates with the ferroelectric capacitor, a lower electrode of aluminum is deposited on a silicon substrate. The aluminum forms as a polycrystalline layer. The then deposited ferroelectric, usually having a perovskite crystal structure in the bulk, grows in polycrystalline form as well. However, the polycrystalline microstructure of the ferroelectric film degrades the film properties because the grain boundaries are sites for charge segregation and charge decay and the crystalline interfaces are sites for chemical segregation or for formation of secondary phases, all deleterious to device performance. The polycrystalline microstructure has been linked to several critical technology issues: fatigue, aging, lower saturation and remanent polarization, time dependent decay of the polarization, and leakage current.
Several groups have investigated the epitaxial growth of ferroelectric films using a variety of growth processes usually associated with semiconductors. For example, Iijima et al. disclose activated reactive evaporation of ferroelectric films in "Preparation of ferroelectric BaTiO.sub.3 thin films by activated reactive evaporation," Applied Physics Letters, volume 56, 1990, pages 527-529. They grew several structures, including BaTiO.sub.3 on a SrTiO.sub.3 substrate and another with a 100 nm epitaxial Pt between the BaTiO.sub.3 and the SrTiO.sub.3. Such a thin Pt layer was probably required to retain it in the pseudomorphic regime where it would remain epitaxial with the substrate. Less constraint on the design of the lower electrode is desirable.
Davis et al. disclose laser ablation growth of ferroelectric films on LiF substrates in "Epitaxial growth of thin films of BaTiO.sub.3 using excimer laser ablation," Applied Physics Letters, volume 55, 1989, pages 112-114. This growth technique has found recent popularity in growing the high-temperature superconducting copper oxides, YBaCuO and BiSrCaCuO. They, however, provided no guidance on ways of achieving a lower electrical contact. Laser ablation has been used for the growth of epitaxial heterostructures, for example, superconducting YBaCuO on insulating PrBaCuO, as has been disclosed by Hegde et al. in U.S. Pat. application, Ser. No. 07/360,090, filed Jun. 1, 1989 now U.S. Pat. No. 5,087,605, issued Feb. 11, 1992.